Classical compounds and neoclassical compounds are compound words composed from combining forms (which act as affixes or stems) derived from classical Latin or ancient Greekroots. New Latin comprises many such words and is a substantial component of the technical and scientificlexicon of English and other languages, including international scientific vocabulary. For example, bio- combines with -graphy to form biography. A vowel usually facilitates the combination: in biography, the Greek thematic vowel -o-, in miniskirt, the Latin thematic -i-. This vowel is usually regarded as attached to the initial base (bio-, mini-) rather than the final base (-graphy, -skirt), but in Greek-derived forms it is sometimes shown as attached to the final base (-ography, -ology). If, however, the final base begins with a vowel (for example, -archy as in monarchy), the mediating vowel has traditionally been avoided (not *monoarchy), but in recent coinages it is often kept and generally accompanied by a hyphen (auto-analysis, bio-energy, hydro-electricity, not *autanalysis, *bienergy, *hydrelectricity).

Classical compounds represent a significant source of Neo-Latin vocabulary. Moreover, since these words are composed from classical languages whose prestige is or was respected throughout the West European culture, these words typically appear in many different languages. Their widespread use makes technical writing generally accessible to readers who may only have a smattering of the language in which it appears.

Not all Western European languages have been equally receptive to classical technical compounds. German, for instance, has historically attempted to create its own technical vocabulary from native elements. Usually, these creations are German calques on the international vocabulary, such as Wasserstoff for hydrogen. Like any exercise in language prescription, this endeavour has been only partially successful, so while official German may still speak of a Fernsprecher, public telephones will be labelled with the internationally recognized Telefon.

Thus, for example, Ancient Greek σφιγξ becomes English (and Latin) sphinx. Exceptions to these romanizing rules occur, such as leukemia (leukaemia); compare leukocyte, also leucocyte. In Latin, and in the target languages, the Greek vowels are given their classical values rather than their contemporary values in demotic Greek.

Ancient Greek words often contain consonant clusters which are foreign to the phonology of contemporary English and other languages that incorporate these words into their lexicon: diphthong; pneumatology, phthisis. The traditional response in English is to treat the unfamiliar cluster as containing one or more silent letters and suppress their pronunciation, more modern speakers tend to try and pronounce the unusual cluster. This adds to the irregularities of English spelling; moreover, since many of these words are encountered in writing more often than they are heard spoken, it introduces uncertainty as to how to pronounce them when encountered.

Classical compounds frequently vary their stressed syllable when suffixes are added: ágriculture, agricúltural. This also gives rise to uncertainty when these words are encountered in print. Once a classical compound has been created and borrowed, it typically becomes the foundation of a whole series of related words: e.g. astrology, astrological, astrologer/astrologist/astrologian, astrologism.

Mainstream medical New Latin and ISV pronunciation in English is not the same as Classical Latin pronunciation. Like Ecclesiastical Latin, it has a regularity of its own, and individual sounds can be mapped or compared. Although the Classical Latin pronunciation of venae cavae would be approximately /ˈwɛnaɪˈkɑːvaɪ/, the standard English medical pronunciation is /ˈviːniːˈkeɪviː/.

English began incorporating many of these words in the sixteenth century; geography first appeared in an English text in 1535. Other early adopted words that still survive include mystagogue, from the 1540s, and androgyne, from the 1550s. The use of these technical terms predates the scientific method; the several varieties of divination all take their names from Classical compounds, such as alectryomancy, divination by the pecking of chickens.

Not all English writers have been friendly to the inflow of Classical vocabulary. The Tudor period writer Sir John Cheke wrote:

I am of this opinion that our own tung should be written cleane and pure, unmixt and unmangeled with borrowing of other tunges; wherein if we take not heed by tiim, ever borowing and never paying, she shall be fain to keep her house as bankrupt.

Many such words, such as thermometer, dinosaur, rhinoceros, and rhododendron, are thoroughly incorporated into the English lexicon and are the ordinary words for their referents. Some are prone to colloquial shortening; rhinoceros often becomes rhino, a situation which may give rise to ambiguity when someone moves from speaking of rhinoceroses to rhinoviruses, being unclear whether the speaker is talking about a Rhino virus, or a Rhinovirus. The binomial nomenclature of taxonomy and biology is a major source for these items of vocabulary; for many unfamiliar species that lack a common English name, the name of the genus becomes the English word for that life form.

In the metric system, prefixes that indicate multipliers are typically Greek in origin, such as kilogram, while those that indicate divisors are Latin, as in millimeter: the base roots resemble Greek words, but in truth are neologisms. These metric and other suffixes are added to native English roots as well, resulting in creations such as gigabyte. Words of mixed Latin and Greek lineage, or words that combine elements of the classical languages with English – so-called hybrid words – were formerly castigated as "barbarisms" by prescriptionist usage commentators; this disapproval has mostly abated. Indeed, in scientific nomenclature, even more exotic hybrids have appeared, such as for example the dinosaur Yangchuanosaurus. Personal names appear in some scientific names such as Fuchsia.

Classical compounds are sometimes used to lend grandeur or the impression of scientific rigour to humble pursuits: the study of cosmetology will not help anyone become an astronaut. Compounds along these models are also sometimes coined for humorous effect, such as odontopodology, the science of putting your foot into your mouth. These humorous coinages sometimes take on a life of their own, such as garbology, the study of garbage.

Some classical compounds form classical plurals, and are therefore irregular in English. Others do not, while some vacillate between classical and regular plurals.

There are hundreds of classical compounds in English and other European languages. As traditionally defined, they cannot stand alone as free words, but there are many exceptions to this rule, and in the late 20th century such forms are increasingly used independently: bio as a clipping of biography, telly as a respelt clipping of television. Most classical compounds translate readily into everyday language, especially nouns: bio- as ‘life’ -graphy as ‘writing, description’. Because of this, the compounds of which they are part (usually classical or learned compounds) can be more or less straightforwardly paraphrased: biography as ‘writing about a life’, neurology as ‘the study of the nervous system’. Many classical compounds are designed to take initial or final position: autobiography has the two initial or preposed forms auto-, bio-, and one postposed form -graphy. Although most occupy one position or the other, some can occupy both: -graph- as in graphology and monograph; -phil- as in philology and Anglophile. Occasionally, the same base is repeated in one word: logology the study of words, phobophobia the fear of fear.

Forms that come first include: aero- air, crypto- hidden, demo- people, geo- earth, odonto- tooth, ornitho- bird, thalasso sea. Many have both a traditional simple meaning and a modern telescopic meaning: in biology, bio- means ‘life’, but in bio-degradable it telescopes ‘biologically’; although hypno- basically means ‘sleep’ (hypnopaedia learning through sleep), it also stands for ‘hypnosis’ (hypnotherapy cure through hypnosis). When a form stands alone as a present-day word, it is usually a telescopic abbreviation: bio biography, chemo chemotherapy, hydro hydroelectricity, metro metropolitan. Some telescoped forms can be shorter than the original classical compounds: gynie is shorter than gyneco- and stands for both gynecology and gynecologist; anthro is shorter than anthropo- and stands for anthropology. Forms that come second include: -ectomy cutting out, -graphy writing, description, -kinesis motion, -logy study, -mancy divination, -onym name, -phagy eating, -phony sound, -therapy healing, -tomy cutting. They are generally listed in dictionaries without the interfixed vowel, which appears however in such casual phrases as ‘ologies and isms’.

In Greek and Latin grammar, combining bases usually require a thematic or stem-forming vowel. In biography, from Greek, the thematic is -o-; in agriculture, from Latin, it is -i-. In English, which does not inflect in this way and has no native thematic vowels, an element like -o- is an imported glue that holds bases together. Its presence helps to distinguish classical compounds like biography and agriculture from vernacular compounds like teapot and blackbird. Generally, English has acquired its classical compounds in three ways: through French from Latin and Greek, directly from Latin and Greek, and by coinage in English on Greek and Latin patterns. An exception is schizophrenia, which came into English through German, and is therefore pronounced ‘skitso’, not ‘skyzo’. The classical compounds are as much a part of English as of Latin and Greek, and as much a part of French, Spanish, Italian, and any other language that cares to use them. They are an international resource.

From the Renaissance until the mid-20th century, the concept of derivational purity has generally regulated the use of classical compounds: Greek with Greek, Latin with Latin, and a minimum of hybridization. Biography is Greek, agriculture Latin, but television is a hybrid of Greek tele- and Latin -vision (probably so coined because the ‘pure’ form telescope had already been adopted for another purpose). Most dictionaries follow the Oxford English Dictionary in using combining form (comb. form) to label such classical elements, but the name is not widely known. In appendices to dictionaries and grammar books, classical compounds are often loosely referred to as roots or affixes: ‘a logo …, properly speaking, is not a word at all but a prefix meaning word and short for logogram, a symbol, much as telly is short for television’ (Montreal Gazette, 13 Apr. 1981). They are often referred to as affixes because some come first and some come last, but if they were affixes, a word like biography would have no base whatever. While affixes are grammatical (like prepositions), classical compounds are lexical (like nouns, adjectives, and verbs): for example, bio- translates as a noun (life), -graphy as a verbal noun (writing). They are also often loosely called roots because they are ancient and have a basic role in word formation, but functionally and often structurally they are distinct from roots: the -graph in autograph is both a root and a classical compound, while the -graphy in cryptography consists of root -graph- and suffix -y, and is only a classical compound.

Generally, classical compounds were a closed system from the 16c to the earlier 20c: the people who used them were classically educated, their teachers and exemplars generally took a purist's view on their use, contexts of use were mainly technical, and there was relatively little seepage into the language at large. However, with the decline of classical education and the spread of technical and quasi-technical jargon in the media, a continuum has evolved, with at least five stages:

In the later 20c, many forms have cut loose from ancient moorings: crypto- as in preposed Crypto-Fascist and pseudo- as in pseudoradical; postposed -meter in speedometer, clapometer. Processes of analogy have created coinages like petrodollar, psycho-warfare, microwave on such models as petrochemical, psychology, microscope. Such stunt usages as eco-doom, eco-fears, eco-freaks, common in journalism, often employ classical compounds telescopically: eco- standing for ecology and ecological and not as used in economics. In such matters, precision of meaning is secondary to compactness and vividness of expression.

In recent years, the orthography of many word forms has changed, usually without affecting pronunciation and stress. The same spoken usage may be written micro-missile, micro missile, micromissile, reflecting the same uncertainty or flexibility as in businessman, business-man, business man. When used in such ways, classical compounds are often telescopic: Hydro substation Hydro-Electricity Board substation, Metro highways Metropolitan highways, porno cult pornography cult.

The mix of late 20c techno-commercial coinages includes three groups of post- and non-classical forms: (1)Established forms: econo- from ‘economic’, as in econometric, Econo-Car, mini- from ‘miniature’, as in miniskirt, mini-boom, -matic from ‘automatic’, as in Adjustamatic, Instamatic, Stackomatic. (2) Less established forms, often created by blending: accu- from ‘accurate’, as in Accuvision; compu- from ‘computer’, as in Compucorp; docu- from ‘documentary’, as in docudrama; perma- from ‘permanent’, as in permafrost and permaban; dura- from ‘durable’, as in Duramark. (3) Informal vernacular material in pseudo-classical form: Easibird, Healthitone, Redi-pak, Relax-a-Cisor (relax, exerciser).

Word forms exist for describing relations or interactions between two nations or societies, such as "Anglo-French" (England and France), Franco-Italian (France and Italy), Greco-Turkish (Greece and Turkey), Russo-Japanese (Russia and Japan), Sino-Cambodian (China and Cambodian), Americo-Liberian (the United States and the African nation of Liberia). In theory, at least, word forms exist to describe every conceivable pairing, although some (Peruvo-Ugandan? Papuo-Icelandic?) may tend more to the fabulous than the actually useful forms. Note that it could be argued that Franco-English, Italo-French, Turco-Grecian, and so forth are equally valid ways of describing the relationship or interactions. The term Afro-American was formerly used to describe a Black American, but now it is deemed more politically correct not to use the Afro- combining form and instead the term African American is used.

In East Asia, a similar role to Latin and Greek has been played by Chinese, with non-Chinese languages both borrowing significant number of words from Chinese, and using morphemes borrowed from Chinese to coin words, particularly formal or technical language. See Sino-Japanese vocabulary, Sino-Korean vocabulary, and Sino-Vietnamese vocabulary for discussion.

The coinage of new native terms on Chinese roots is most notable in Japanese, where it is referred to as wasei kango(和製漢語?, Japanese-made Chinese-words). Many of these have been subsequently borrowed into Chinese, Korean, and Vietnamese, with the same (or corresponding) characters being pronounced differently according to language, just as happens in European languages – compare English biology and French biologie.